I knew that Meterology course would prove useful someday.

Fog is not high enough to generate lightning. It's mostly an upper atmosphere pheonomenon, which is why you only see it in the really big thunderheads.

Mid-range clouds seem rare because there is soooo much atmosphere up there. Even if it was an even distrobution of cloud heights, ones within ~100 ft of the ground would seem unusual.

Fog is kinda an exemption based on the way it forms. Normally, terestrial moisture has to travel a ways into the atomosphere before adiabatic cooling causes water droplets to form. In the case of fog, it is cool enough for the dropplets right at ground level.